This invention relates to the method and apparatus for transmitting and receiving anti-jam radio signals.
Although there have been numerous techniques and apparatuses developed for a secured communication system, the two most commonly used techniques for anti-jam protection can roughly be characterized as band spreading and frequency hopping. In band spreading, the frequency band of transmission is greatly increased by adding a high speed pseudorandom data stream to the basic information data or signal. The resulting high data rate signal is spread over a wide spectrum of frequencies. If the frequency is decoded by a receiver using an identical pseudorandom data stream, the basic information can be extracted. Since very little energy occurs at any narrow band point in the transmitted spectrum, the noise-like signal is difficult to detect or jam without correlation and synchronization with the pseudorandom data stream. The degree of protection is frequently stated as a processing gain which is the ratio of the transmitted band to the information band. Effective utilization of this technique requires very wide band radio transmitters and receivers. This requirement makes band spreading incompatible with most existing radios which are narrow band, not only because of the limitations of components presently available, but also because in the past it has been a desirable design feature to increase selectivity in order to reduce mutual interferences.
Frequency hopping involves the changing of the radio carrier or center frequencies periodically to avoid detection or jamming. The degree of protection is, of course, related to the hopping rate and sequence of frequencies. This is somewhat difficult to state quantitatively. However, it can be appreciated that the sophistication of detection and jamming equipment required to constitute a threat to a communications system randomly hopping every hundred milliseconds would be great. Synchronization of the transmitter and receiver is required as in the case with band spreading. Since the prior art hopping rates are low, synchronization is somewhat easier with the frequency hopping method.
Very fast frequency hopping is not compatible with many existing radios because of their slow tuning times. Once again, this limitation is due to the components available at the time of the design of the equipment.
It thus can be concluded that the two prior art methods of protection are limited by the various radio equipment involved and the degree of protection that is required and are limited to half duplex operation without separate transmitters and receivers.